The present invention relates to an absorption medium for deacidifying a fluid stream which comprises acid gases as impurities, and to a process for using the absorption medium.
In numerous processes of the chemical industry, fluid streams occur which comprise acid gases, for example CO2, H2S, SO2, CS2, HCN, COS, disulfides or mercaptans as impurities. These fluid streams can be, for example, gas streams, such as natural gas, synthesis gas, refinery gas, or reaction gases which are formed in the oxidation of organic materials, for example organic wastes, coal, natural gas or petroleum, or in the composting of waste matters containing organic substances.
Removing the acid gases is of particular importance for different reasons. For example, the content of sulfur compounds in the natural gas must be reduced by suitable treatment measures directly at the natural gas well, since the sulfur compounds, together with the water frequently entrained by the natural gas, also form acids which act corrosively. Therefore, for transporting the natural gas in a pipeline, preset limiting values of the sulfur impurities must be maintained. The acid gases must be removed from the reaction gases formed in the oxidation of organic materials to prevent the emission of gases which can damage the natural environment or affect the climate.
On an industrial scale, aqueous solutions of organic bases, for example alkanolamines, are frequently used as absorption media. When acid gases dissolve, ionic products form from the base and the acid gas constituents. The absorption medium can be regenerated by expansion to a lower pressure, or stripping, with the ionic products back-reacting it to form acid gases and/or the acid gases being stripped off by steam. After the regeneration process the absorption medium can be reused. Preferred alkanolamines used in the removal of acid gas impurities from hydrocarbon gas streams comprise monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diethylethanolamine (DEEA), diisopropylamine (DIPA), aminoethoxyethanol (AEE) and methyldiethanolamine (MDEA).
The known absorption media are highly suitable for deacidifying hydrocarbon streams such as natural gas. Certain problems occur in the treatment of oxygen-containing fluids, for example flue gases. In this case the absorption capacity of the absorption medium is impaired in the long term and is not completely recovered on regeneration. It is probable that the presence of molecular oxygen is responsible for oxidative destruction of the amines present in the absorption medium.
U.S. Pat. No. 3,137,654 proposes adding complexing agents, such as N,N-di-hydroxyethylglycine, N-hydroxyethylethylenediaminetriacetic acid sodium salt or ethylenediaminetetraacetic acid sodium salt, to the absorption medium to slow down the oxidative breakdown of the absorption medium.
U.S. Pat. No. 4,440,731 teaches adding more than 50 ppm of Cu2+ ions, if appropriate mixed with dihydroxyethylglycine, an alkali metal carbonate, an alkali metal permanganate or ammoniumpermanganate and nickel oxide and/or bismuth oxide, to the absorption medium for stabilization.
It is known to use antioxidants to stabilize inanimate organic materials, such as molding compounds or paints, against the action of oxygen, light and/or heat. The mode of action of the antioxidants is based on the fact that the propagation step of a free-radically induced peroxidation reaction is interrupted, for example by the antioxidants transferring an H atom to the intermediate peroxyl free radical. Many antioxidants additionally act as reducing agents. The use of antioxidants to stabilize liquid absorption media has not hitherto been mentioned, since those skilled in the art would expect, in the case of intimate and intensive contact of the absorption medium with oxygen-containing fluids, a direct reaction of the antioxidant with the oxygen, the antioxidant would be consumed, without developing a stabilizing action.